1. Field of the Invention
This invention relates to a frictional engaging clutch control system for an automatic vehicle transmission, more specifically to an improved system for detecting a judder that may occur on the vehicle, and acts to decrease the same.
2. Description of the Related Art
There is sometimes a vibrational disturbance on a vehicle when running on a bumpy road due to the unevenness of the road surface. Similarly, a vibration often called "judder" (self-excited vibration) may occur, which is a kind of self-excited vibration due to control hunting while slip-engaging a frictional engaging clutch such as a start clutch, a torque converter lock-up clutch of the continuously variable type or multi-step transmission. Since such a judder degrades vehicle driving comfort, a vibration should preferably be discriminated or determined whether it is a vibrational disturbance caused by a reason outside of the vehicle such as the rough road surface on which the vehicle is traveling, or it is a judder, and to decrease or suppress the same if it is.
For that purpose, Japanese Laid-Open Patent Application No. Hei 7 (1995)-42,768 proposes to discriminate the occurrence of judder based on rotational speed fluctuations between driven wheels and free wheels of the vehicle. Specifically, the prior art system is based on the vehicle having a chain-transmission mechanism for 4WD which can attenuate the judder occurred on free wheels. Accordingly, the system is configured to discriminate the occurrence of judder from a deviation between the rotational speed fluctuations of the driven wheels and free wheels of the vehicle.
More specifically, the prior art system is configured to determine that a vibration is a judder when the rotational speed fluctuation occurs only on driven wheels, while determining a vibration as a vibrational disturbance when the rotational speed fluctuation occurs both the driven wheels and free wheels. The prior art system changes the lock-up clutch slippage control to decrease or suppress the judder when the vibration is discriminated as being a judder.
However, the prior art system requires to detect both the rotational speed fluctuations of the driven and free wheels, and is disadvantageously complicated in configuration.